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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.enable_if"></a><a class="link" href="enable_if.html" title="enable_if_">enable_if_</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">bool</span> <span class="identifier">B</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">enable_if_</span><span class="special">;</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">bool</span> <span class="identifier">B</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="identifier">enable_if_t</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">enable_if_</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">;</span>
</pre>
<p>
        <span class="bold"><strong>type:</strong></span> If <code class="computeroutput"><span class="identifier">B</span></code>
        is true, then the member <code class="computeroutput"><span class="identifier">type</span></code>
        is defined to be <code class="computeroutput"><span class="identifier">T</span></code>. Otherwise
        there is no member <code class="computeroutput"><span class="identifier">type</span></code>.
      </p>
<p>
        <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span>
        <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">enable_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          The trait has the name <code class="computeroutput"><span class="identifier">enable_if_</span></code>
          (with a trailing underscore) but behaves like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">enable_if</span></code>
          or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if_c</span></code>. The existing trait with
          the name <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code> has a different interface.
        </p></td></tr>
</table></div>
<p>
        <span class="bold"><strong>Examples:</strong></span>
      </p>
<p>
        The following function can be used to destroy each element of an array and
        specially handle arrays of trivially destructible types.
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if_</span><span class="special">&lt;!</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_trivial_destructor</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">destroy</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">ptr</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">)</span>
<span class="special">{</span>
    <span class="keyword">while</span> <span class="special">(</span><span class="identifier">size</span> <span class="special">&gt;</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
        <span class="identifier">ptr</span><span class="special">[--</span><span class="identifier">size</span><span class="special">].~</span><span class="identifier">T</span><span class="special">();</span>
    <span class="special">}</span>
<span class="special">}</span>

<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if_</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_trivial_destructor</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">&gt;::</span><span class="identifier">type</span>
<span class="identifier">destroy</span><span class="special">(</span><span class="identifier">T</span><span class="special">*,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)</span> <span class="special">{</span> <span class="special">}</span>
</pre>
<p>
        <span class="bold"><strong>Compiler Compatibility:</strong></span> All current compilers
        are supported by this trait. The type alias <code class="computeroutput"><span class="identifier">enable_if_t</span></code>
        is only available if the compiler supports template aliases.
      </p>
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<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams,
      Frederic Bron, Steve Cleary, Beman Dawes, Glen Fernandes, Aleksey Gurtovoy,
      Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander
      Nasonov, Thorsten Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert
      Stewart and Steven Watanabe<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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